Powdered hydrated lime, also known as slaked lime, is understood to mean a set of solid particles consisting mainly of calcium hydroxide Ca(OH)2.
This slaked lime may obviously contain impurities, such as magnesium oxide or hydroxide, silica, alumina, etc., in a quantity of a few tens of grams per kilo. In general, the particle size of this powdered material is on the whole less than 1 mm and often less than 250 μm. Slaked lime may contain free water, that is to say water that is not chemically bound to the compound, in a quantity of up to around 50 g/kg.
Powdered slaked lime is used in many applications, in particular as a neutralizing agent for acid compounds (HCl, SO2, HF, SO3 . . . ) contained in flue gases. In this “dry” method, which is simple and inexpensive, the powdered lime, which is used as an absorbent, is brought into direct contact with the gases to be purified. However, the neutralization reaction between gas and solid is not easy, and a large excess of calcareous reagent with respect to the quantity of acid to be neutralized is often necessary in order to meet increasingly strict emission standards. This excess of reagent poses the problem in particular of additionally generating by-products or residues which have to be treated downstream.
To reduce the excess of reagent or absorbent that has to be used, many products based on powdered slaked lime have been developed in order to obtain a better capability for trapping acid gases using the “dry method”.
In particular, it is known to promote the ability of conventional slaked limes to trap gaseous HCl by combining said conventional slaked limes with an additive, such as alkali metal hydroxides or carbonates or alkali metal chlorides [CHEN, D. et coll., International Academic Publishers, 1999, pp. 337-342]. In said document, the additive in question is added to the slaking water for the quicklime during the preparation of the hydrated lime. The authors note an improvement in performance with regard to reducing gaseous HCl for the slaked lime thus obtained, compared to when the additive is absent, at use temperatures above 200° C. On the other hand, no effect in terms of reducing SO2 is mentioned.
Other works, as presented in the U.S. Pat. No. 4,604,269, recommend the addition, to the slaking water for the quicklime, of additives such as sodium hydroxide (NaOH) in a quantity of 5% to 10% by weight, with respect to the quicklime, or alternatively chlorides, such as calcium chloride (CaCl2). The slaked lime thus obtained promotes the removal of sulphur from flue gases at “low” temperature, that is to say less than 230° C., and preferably less than 175° C. This is because the action of the additive appears when the use temperature of the absorbent differs from the dew point by less than 25° C., preferably less than 10° C. Under these conditions, the additive has the effect of making the absorbent deliquescent in the present of moisture, which promotes the presence of a liquid film at the solid/gas interface and improves the trapping of SO2.
The document WO 88/09203 again takes up this concept of adding an alkali metal compound, such as NaOH or chlorides such as CaCl2, to the slaking water for the quicklime. The quantities and the effect of these additives are not really discussed. The first is said to have the aim of increasing the basicity of the absorbent, and the second is said to have the aim of retaining the water, as in the case of U.S. Pat. No. 4,604,269 mentioned above.
The document [Method for producing reactive Coolside sorbent—Production of reactive sorbent for cool-size process—by hydrating quicklime with water containing sodium chloride aqueous solution, Research Disclosure, 1988, 295(898), No. 29564, ISSN: 03744353], confirms the positive effect on sulphur removal under conditions close to saturation (preferably less than 20° C. above the dew point) of additives such as Na2CO3, NaOH, CaCl2 and especially NaCl, present in a quantity of more than 5% by weight of the absorbent, by addition to the slaking water. However, the slaked lime thus modified has a BET specific surface area that is less than that of conventional hydrated lime obtained in the absence of additive. Under the use conditions studied, the use of organic additives, such as sugars and surfactants, does not improve the sulphur-removing properties of slaked limes.
In the present text, “first generation” will be used to denote the absorbents of the prior art which are based on slaked lime, the ability of which in respect of trapping acid gases has been improved in comparison with a “conventional” or “standard” slaked lime by adding an additive of the aforementioned type, that is to say a “chemical” modification.
Another class of absorbents based on slaked lime also exists, the ability of which in respect of trapping acid gases is greater than that of a conventional hydrated lime. The advantage of these absorbents then results from a modification of the physical properties, in this case the texture, namely a greater BET specific surface area and/or a greater BJH pore volume. These absorbents will be called “second generation” absorbents, resulting from a “physical” modification, see [OATES, J. A. H., Lime and limestone, Weinheim: Wiley-VCH, 1998, 455, pp. 219-221].
There is known, for example from the document WO97/14650, a powdered lime composition comprising calcium hydroxide particles having a BET specific surface area that is greater than 25 m2/g and a BJH total pore volume obtained from nitrogen desorption that is at least 0.1 cm3/g.
Said document describes in particular a product based on hydrated lime, the BJH pore volume and the BET specific surface area of which are markedly more developed than those of a standard calcium hydroxide. The ability of such a hydrated lime to trap acid gases is much improved compared to a conventional hydrated lime but also compared to a first-generation slaked lime. The second-generation hydrated lime according to international patent application WO97/14650 is at present considered to be the calcareous reagent with the best performance when it comes to trapping acid gases by the dry method, within a wide range of operating conditions.
However, this second-generation lime does not exhibit such a high increase in the aforementioned trapping performance in respect of all the acid gases that are potentially present in flue gases. In particular, the increase in performance of these second-generation absorbents for reducing the amount of sulphur-containing compounds such as SO2 is not as great as that relating to the reduction in HCl.